The electrical conductance of single- and double wall carbon nanotube systems was measured by a mechanically controllable break- junction technique using freestanding nanotubes not subject to any chemical modification. For metallic single-wall carbon nanotubes, two channels with transmission coefficient of o.88 contribute to ballistic electronic transport at room temperature. In double- wall carbon nanotubes, one metallic channel contributes to ballistic electronic transport and additional field -and temperature- dependent two- channel contributions were .observed
